The present invention relates generally to acousto-optic devices, and more particularly to an electrically controlled multiple dispersion (zoom) device.
Acousto-optic devices use acoustic power to perform optical manipulations. In general, the interaction between light and sound occurs through the elasto-optic effect. The acoustic wave establishes a phase grating within the material which causes the entering light beam to be diffracted. By varying the applied acoustic frequency, the grating spacing is varied, allowing the optical beam to be manipulated. Cells of this type are referred to as Bragg cells and may differ according to the type of material used. Bragg cells fabricated from materials whose indices of refraction are isotropic are referred to as normal isotropic Bragg cells, whereas cells fabricated from birefringent materials are called anisotropic Bragg cells. The distinction is an important one and leads to different design considerations.
In a normal Bragg cell, the angle of incidence is equal to the angle of diffraction. Changing the diffraction angle--for example, by changing the grating spacing--requires an equal change in the incident angle relative to the acoustic wavefront. Hence, either the input beam direction or the acoustic wavefront must be steered to maintain the phase matching condition--that is, to conserve momentum. In an anistropic Bragg cell, the input angle remains essentially fixed over a wide range of acoustic frequencies around the design center, and the optical beam can undergo considerable manipulation without much change in the input beam direction. The latter clearly has an advantage over the isotropic Bragg cell and is now widely used as the preferred mechanism.
There are important situations encountered when it is desirable to examine a spectrum at low resolution and subsequently choose a portion of that spectrum to be examined at higher resolution. The latter spectrum should be capable of being chosen arbitrarily to be anywhere within the low resolution spectrum. To accomplish this with a single detector array is difficult if we wish to use the entire array for both the high and low resolution measurements. One must have a device whose dispersion can be changed for one or the other measurement. Conventional "dispersive devices" such as gratings or prisms have dispersions that are fixed by their geometric design, and changes must be made by either changing their geometry, or by switching from one grating or prism to another. The AODLF (Acousto-Optic Dispersive Light Filter), which is an electronically controlled dispersive device, also has a dispersion that is not easily changed once the design has been set.
An AODLF is an acousto-optic spectroscopic device that exploits the optical birefringence properties of certain unique acousto-optic crystals, such as thallium arsenic selenide. The structure and operation of an AODLF is disclosed in U.S. Pat. Nos. 4,639,092; 4,653,869 and 4,886,346, hereby incorporated by reference.
An AODLF functions similar to a conventional diffraction grating. But in an AODLF, the diffraction grating or spacing is electronically determined by the frequency of the acoustic signal applied to the AODLF. A crucial difference between a conventional diffraction grating and an AODLF is that the AODLF operates as a birefringent device, in which the polarization of the diffracted light is rotated 90.degree. with respect to that of the incident light, and the refractive indices are different in the acousto-optical crystal for the incident light and the diffracted light.
The following U.S. patents are of interest.
4,653,869--Gottlieb et al issued Mar. 31, 1987 PA1 4,639,092--Gottlieb et al issued Jan. 27, 1987 PA1 4,886,346--Gottlieb et al issued Dec. 12, 1989 PA1 3,437,951--Dailey issued Apr. 8, 1969 PA1 3,502,879--Vallese issued Mar. 24, 1970 PA1 3,615,449--Greenaway issued Oct. 26, 1971.
The patent to Gottlieb et al teaches a method and apparatus for increasing the angular aperture of an AODLF in which acoustic waves of differing frequencies are launched into a crystal at differing angles to each other. Input light waves phase match with corresponding waves of the acoustic frequencies so that the angular aperture is enlarged.
The patent to Dailey teaches a laser, wherein a prism is located between the laser generator and two reflective surfaces. The beam through the prism may be refracted at different angles when the RF signal connected to the prism is varied. The patent to Vallese teaches a laser device, in which the spectrum passing through an ultrasonic cell is diffracted by varying the electronic signal to the electronic cell. The patent to Greenaway teaches a method of generating high area density periodic arrays by diffraction imaging employing a wedge prism.